This invention is directed to an improved switch particularly adapted to be used as a coaxial cable switch for cable television systems because of its superior isolation of different inputs.
Switches capable of handling high frequency signals, i.e., up to 300 MHz, are frequently ineffective because of shielding problems. Either the switch itself is not sufficiently shielded from the surrounding environment or the inputs to the switch are not adequately shielded from one another. Such switches may be used, for example, in selecting inputs between cable and antenna, or between different cable inputs. In CATV systems these are known as A-B switches.
Presently available input switches have a rather low isolation between inputs, and are also of rather complicated construction. Therefore, it is desired to make a switch that is simple and economical in construction, and also provides high isolation between inputs.
Many high frequency input switches only deal with shielding the inputs from the environment, and not shielding one input from another input. This is generally done by a shielding chamber which encloses all of the terminals, including both input terminals and the output terminal, i.e., the housing merely serves as a high frequency shielded chamber. Usually, the chamber is designed so that it forms a waveguide below cutoff for the principal waveguide mode in the operating frequency range of the switch. This merely isolates the inputs in the switch. It does not isolate one input from another input.
U.S. Pat. No. 3,648,000 discloses a switch for selecting between high frequency cable TV inputs, and teaches a grounding means for providing increased isolation of the respective cables. It isolates by grounding the unused input contact and the contact member which makes the connection between the input and the output. No other isolation of the unused terminals is provided for or taught. This patent states that 75 decibels (db) of isolation are provided with such a construction.